In order to be able to produce ever finer structures, various ways of increasing the resolution capability of the projection lenses are pursued. As is known, the resolution capability can be increased by increasing the image-side numerical aperture (NA) of the projection lens. Another measure is to operate with ever shorter wavelengths of the electromagnetic radiation used.
Improving the resolution by increasing the numerical aperture can have some disadvantages. One disadvantage is that the depth of focus (DOF) that can be obtained generally decreases as the numerical aperture increases. This can be disadvantageous because, for example for reasons of the obtainable flatness of the substrates to be structured and mechanical tolerances, a depth of focus of the order of magnitude of at least one micrometer is typically desirable. Therefore, systems have been developed which operate with moderate numerical apertures and obtain the increase in the resolution capability substantially via the short wavelength of the electromagnetic radiation used from the extreme ultraviolet range (EUV). In the case of EUV lithography with operating wavelengths around 13.5 nm, for example given numerical apertures of NA=0.1, theoretically a resolution of the order of magnitude of 0.1 μm can be achieved with typical depths of focus of the order of magnitude of approximately 1 μm.
As is known, radiation from the extreme ultraviolet range generally can no longer be focused with the aid of refractive optical elements, since the short wavelengths are typically absorbed by known optical materials that are transparent at higher wavelengths. Therefore, for EUV lithography, mirror systems are used in which a plurality of imaging, i.e. concave or convex, mirrors provided with reflective layer arrangements (reflection coatings) are arranged between the object plane and the image plane. The layer arrangements used are typically multilayer coatings, that is to say layer arrangements having a sequence of individual layers.
In the case of mirror systems for microlithography which are designed for radiation from the EUV wavelength range, the mirrors used for the exposure or imaging of a mask into an image plane desirably have a high reflectivity, since firstly the product of the reflectivity values of the individual mirrors determines the total transmission of the projection exposure apparatus and since secondly EUV light sources are usually limited in terms of their light power.
Mirrors for the EUV wavelength range around 13 nm having high reflectivity values are known for example from DE 101 55 711 A1. The mirrors described therein have a layer arrangement applied on a substrate and having a sequence of individual layers, wherein the layer arrangement includes a plurality of layer subsystems each having a periodic sequence of at least two individual layers of different materials that form a period. The number of periods and the thickness of the periods of the individual subsystems decrease from the substrate toward the surface. Such mirrors have a reflectivity of greater than 30% in an angle of incidence interval between 0° and 20°.
In this case, the “angle of incidence” denotes the angle between the direction of incidence of a ray and the normal to the surface of the mirror at the point where the ray impinges on the mirror. Since a respective light beam impinges on a mirror from each object point, superimposition of the light beams occurs at each point of the mirror surface used. In this case, at each point of a mirror surface, rays are incident at different angles of incidence. In this application, the term “angle of incidence interval” denotes the angle interval between the largest and the smallest angle of incidence respectively considered at a point of the mirror. The term “mirror having the largest angle of incidence interval” accordingly denotes that mirror whose mirror surface encompasses the surface point having the largest angle of incidence interval.
In the case of the layers mentioned above, however, the reflectivity in the angle of incidence interval specified is not constant, but rather it varies. A variation of the reflectivity of a mirror over the angles of incidence can be problematic particularly for the use of such a mirror at locations with high angles of incidence and with great variation of the angles of incidence (i.e. large angle of incidence interval) in a projection lens for microlithography, since such a variation can lead for example to an excessively large variation of the pupil apodisation of such a projection objective. In this case, the “pupil apodisation” is a measure of the spatial intensity fluctuation over the exit pupil of a projection lens, that is to say an energetic system characteristic value.
In this respect, it is desirable for mirrors which have a broadband reflection effect in the angle space. This angular broadband nature relates to the ability of a layer arrangement to have high reflectivity which varies only as little as possible for an angle of incidence range that is as large as possible.
Since, furthermore, the radiation sources provided for the EUV range, e.g. plasma sources, are typically not narrowband, but rather emit radiation over a relatively wide wavelength range around a main wavelength, a spectral broadband nature, i.e. a broadband nature of the reflection effect of the layer arrangement in the wavelength space, is desirable.
For the design of the entire optical system it should typically be ensured that both energetic system characteristic values, such as e.g. the pupil apodisation and the total transmission, and the aberrations important for the imaging quality, in particular the chromatic aberrations, remain within the scope of predetermined specifications.